1. Field of the Invention
The present invention relates to calibrating a micro-actuator that positions a magnetic head in a hard disk drive.
2. Background Information
Hard disk drives contain one or more magnetic heads coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Typically, magnetic heads have a write element for magnetizing the disks and a separate read element for sensing the magnetic field of the disks. The read element is typically constructed from a magneto-resistive material. The magneto-resistive material has a resistance that varies with the magnetic fields of the disk. Heads with magneto-resistive read elements are commonly referred to as magneto-resistive (MR) heads.
Each head is embedded in a slider. The slider mechanically couples to an actuator arm by a head suspension assembly. The head suspension assembly includes a load beam connected to the actuator arm by a spring or hinge coupling. The slider is attached to a flexure arm and the flexure is attached to the load beam to form a head gimbal assembly (HGA). The head gimbal assembly includes the head suspension assembly, the flexure and the slider. Each HGA in a hard disk drive couples to an actuator arm by the hinge coupling. The actuator arms rigidly couple to a voice coil motor that moves the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surfaces of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks and to different sectors of each track.
A suspension interconnect extends along the length of the flexure and connects the head to a preamplifier. The suspension interconnect typically includes a pair of conductive write traces and a pair of conductive read traces.
The Tracks Per Inch (TPI) in hard disk drives is rapidly increasing, leading to smaller and smaller track positional tolerances. The track position tolerance, or the offset of the magnetic head from a track, is monitored by a signal known as the head Positional Error Signal (PES).
Track Mis-Registration (TMR) occurs when a magnetic head loses the track registration. This often occurs when the disk surface bends up or down. TMR is often a statistical measure of the positional error between a magnetic head and the center of an accessed track.
Today, the bandwidth of the servo controller feedback loop, or servo bandwidth, is typically in the range of 1.1 KHz.
Extending servo bandwidth, increases the sensitivity of the servo controller to drive the voice coil actuator to ever finer track positioning. Additionally, it decreases the time for the voice coil actuator to change track positions.
However, extending servo bandwidth is difficult, and has not significantly improved in years. As track densities increase, the need to improve track positioning, and servo bandwidth, increases. One answer to this need involves integrating a micro-actuator into each head gimbal assembly. These micro-actuators are devices typically built of piezoelectric composite materials, often including lead, zirconium, and tungsten. The piezoelectric effect generates a mechanical action through the application of electric power. The piezoelectric effect of the micro-actuator, acting through a lever between the slider and the actuator arm, moves the magnetic head over the tracks of a rotating disk surface.
The micro-actuator is typically controlled by the servo-controller through one or two wires. Electrically stimulating the micro-actuator through the wires triggers mechanical motion due to the piezoelectric effect. The micro-actuator adds fine positioning capabilities to the voice coil actuator, which effectively extends the servo bandwidth. The single wire approach to controlling one micro-actuator provides a DC (direct current) voltage to one of the two leads of the piezoelectric element. The other lead is tied to a shared ground. The two wire approach drives both leads of one micro-actuator.
There are two approaches to integrating the micro-actuator into a head gimbal assembly. Embedding the micro-actuator between the slider and the load beam, creates a co-located micro-actuator. Embedding the micro-actuator into the load beam, creates a non co-located micro-actuator. The non co-located micro-actuators tend to consume more power, requiring higher driving voltages than the co-located micro-actuators.
A problem arises with integrating micro-actuators into hard disk drives. The micro-actuator devices may vary greatly from part to part. When integrated, the assemblies may respond differently than the isolated micro-actuators. The integrated micro-actuators may also vary significantly at different operating temperatures. A method is needed for measuring the micro-actuator stroke sensitivity when integrated into the hard disk drive. The actuator stroke sensitivity is an estimate of how far the micro-actuator moves the magnetic head at a given voltage of stimulus applied to the micro-actuator.
A second problem arises when integrating micro-actuators into hard disk drives with multiple disk surfaces. Each of the micro-actuators requires its leads to be controlled by the servo-controller. These leads are coupled to wires, which must traverse the main flex circuit to get to the bridge flex circuit. The bridge flex circuit provides electrical coupling to the leads of the micro-actuator.
The main flex circuit constrains many components of the actuator arm assembly within a voice coil actuator. If the shape or area of the main flex circuit is enlarged, changes are required to many of the components of the actuator arm assembly and possibly the entire voice coil actuator. Changing many or most of the components of an actuator arm assembly, leads to increases in development expenses, retesting and recalibrating the production processes for reliability, and inherently increases the cost of production.
The existing shape and surface area of the main flex circuit has been extensively optimized for pre-existing requirements. There is no room in the main flex circuit to run separate control wires to each micro-actuator for multiple disk surfaces. This has limited the use of micro-actuators to hard disk drives with only one active disk surface.